Ensure CI always validate builtin test cases

Install necessary Python packages to satisfy the dependency of builtin
test cases including pip3.

TODO: it takes much longer time to set up Python packages rather than
compiling and validating dcurl itself.

.travis.yml

@@ -4,5 +4,13 @@ compiler:
   - clang
   - gcc
 
+before_install:
+  - sudo apt-get install -y build-essential
+  - sudo apt-get install -y clang  
+  - sudo apt-get install -y python3-pip
+  - sudo apt-get install -y python3-cffi
+  - sudo pip3 install --upgrade pip setuptools
+  - sudo pip3 install pyota
+
 script:
-  - make
+  - make check